Simplify virt_viewer_auth_libvirt_credentials() return value logic
    
    As the function should return < 0 in error cases, let's explicitly
    return/set the return value to -1 in error cases. Otherwise, the
    function will return 0.
    
    This patch also fixes a regression introduced by (a5ce2ed3).
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1142742